The exhaust gas of diesel engines, gasoline engines, and other internal combustion engines includes, for example, carbon monoxide (CO), unburned fuel (HC), nitrogen oxides (NOX), particulate matter (PM), and other constituents. The internal combustion engines are mounted with exhaust purification systems for removing these constituents.
As one method for removing nitrogen oxides, arrangement of an NOX storage reduction catalyst in an engine exhaust passage has been proposed. The NOX storage reduction catalyst stores NOX when the air-fuel ratio of the exhaust gas is lean. When the storage amount of the NOX reaches an allowable amount, the air-fuel ratio of the exhaust gas may be made rich or the stoichiometric air-fuel ratio so that the stored NOX is released. The released NOX is reduced to N2 by the carbon monoxide or other reducing agent which is contained in the exhaust gas.
Japanese Patent Publication (A) No. 2000-314311 discloses a purification system arranging a purification catalyst of nitrogen oxides in an exhaust gas flow path of the internal combustion engine. The nitrogen oxide purification catalyst has a precious metal and a nitrogen oxide trapping material. It is disclosed that the nitrogen oxide purification catalyst can trap nitrogen oxides as NO2 by a higher air-fuel ratio than the stoichiometric air-fuel ratio. Further, the trapping material of nitrogen oxides traps SOX, but it is disclosed that by rendering the atmosphere a reducing one, the trapped SOX can be removed. Further, it is disclosed that the temperature for removing the trapped SOX is preferably 500° C. or more.
The exhaust gas of an internal combustion engine sometimes contains sulfur oxides (SOX). An NOX storage reduction catalyst stores SOX at the same time as storing NOX. If SOX is stored, the storable amount of NOX falls. In this way, the NOX storage reduction catalyst suffers from so-called “sulfur poisoning”. To eliminate sulfur poisoning, sulfur poisoning recovery treatment is performed for releasing the SOX. In the sulfur poisoning recovery treatment, the NOX storage reduction catalyst is raised in temperature and, in that state, the air-fuel ratio of the exhaust gas is made rich or the stoichiometric air-fuel ratio to release the SOX.
At the time of sulfur poisoning recovery treatment of the NOX storage reduction catalyst, the SOX is released into the atmosphere. If the release speed of the SOX is large, a large amount of SOX ends up being released in a short time, so odor and other problems arise.
On the other hand, an NOX storage reduction catalyst suffers from thermal degradation. If thermal degradation occurs, for example, the NOX storable amount is decreased. Thermal degradation proceeds faster the higher the temperature of the NOX storage reduction catalyst. When performing sulfur poisoning recovery treatment, the temperature elevated state continues for a long time. For this reason, at the time of sulfur poisoning recovery treatment, thermal degradation proceeds relatively fast.
In the prior art, the target temperature and the regeneration time of the NOX storage reduction catalyst are set in advance. During this regeneration time, the sulfur poisoning recovery treatment was performed while maintaining the target temperature. Alternatively, the SOX release speed may be detected by using a map using the fuel injection amount and temperature etc. in the combustion chambers as functions. The SOX release amount can be calculated from the SOX release speed. However, the SOX release speed which is detected by the prior art includes relatively large error. For this reason, at the time of sulfur poisoning recovery treatment, there was a possibility that the NOX storage reduction catalyst would be exposed to a higher temperature atmosphere than required and that thermal degradation would excessively proceed. The SOX release speed when performing sulfur poisoning recovery treatment preferably can be precisely detected.